The recipient's mailbox is full.

If you have sent a message to a Yandex address and received a non-delivery report that states the following: “Mailbox size exceeded”, “Mailbox size limit exceeded”, this means that the recipient's mailbox is full and cannot accept your message. This usually happens because the recipient does not use their mailbox often and their mailbox is full.

Try contacting the recipient by another method and report the problem. The recipient will have to log into Yandex.Mail via the web interface to be able to receive emails again.